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[SCG45-02] Deep low-frequency earthquakes with cubic moment-duration relationship
Keywords:low-frequency earthquakes, moment-duration relationship
Moment-duration relationship is one of the important information to understand the source process of slow earthquakes. Ide et al. (2007) proposed the broadband relationship in which the seismic moment is proportional to the duration (M∝D) by compiling the source parameter of various slow events observed in broad time scale. This linear scaling is interpreted as the aseismic diffusion process along the plate interface, which is consistent with the characteristics of observed tremor migration (e.g., Ando et al. 2012). On the other hand, moment-duration relationship of individual slow earthquakes such as low-frequency earthquakes (LFEs) is under debate. The moment-duration relationship of LFEs with small size among various slow events is key information to consider the source evolution process of slow earthquakes. Therefore, we investigated the moment-duration relationship of LFEs in this study.
We estimated durations and moments of LFEs following three procedures. At first, we calculated observed durations and seismic amplitudes by fitting the box-car function with exponential decay to the envelope waveforms applied band-pass filter of 1-20 Hz. Since the estimated parameters are affected by the convolution of Green’s function and the band-pass filtering, in the next step, we estimated the empirical relationship between observed durations and seismic amplitudes and source durations and moments based on the same analysis for synthetic waveforms. Finally, applying empirical functions to observed values, we obtained source duration and moments of LFEs.
We performed this analysis for volcanic LFEs occurring at Zao volcano and tectonic LFEs at western Ehime. We obtained the moment-duration relationship characterized by the moment proportional to the cubic of duration (M∝D3) for both types of LFEs. The obtained relationship is different from the broadband scaling of Ide et al. (2007), which indicates the difference in physical process between small-scale LFEs and large-scale slow events. This result suggests that LFEs do not evolve to large slow earthquakes but they are slow phenomenon with small size triggered by the slow earthquakes with large size such as slow slip events driven by aseismic diffusion. In addition, despite the same moment-duration relationship, we observed that the moments of LFEs are three orders of magnitude smaller than those of ordinary earthquakes with similar durations. We interpret that the rupture velocity and/or stress drop of LFEs smaller than ordinary earthquakes cause the difference in source durations. The reasonable ranges of the rupture velocity and stress drop are 0.1–1 km/s and 2 kPa–1 MPa, respectively.
We estimated durations and moments of LFEs following three procedures. At first, we calculated observed durations and seismic amplitudes by fitting the box-car function with exponential decay to the envelope waveforms applied band-pass filter of 1-20 Hz. Since the estimated parameters are affected by the convolution of Green’s function and the band-pass filtering, in the next step, we estimated the empirical relationship between observed durations and seismic amplitudes and source durations and moments based on the same analysis for synthetic waveforms. Finally, applying empirical functions to observed values, we obtained source duration and moments of LFEs.
We performed this analysis for volcanic LFEs occurring at Zao volcano and tectonic LFEs at western Ehime. We obtained the moment-duration relationship characterized by the moment proportional to the cubic of duration (M∝D3) for both types of LFEs. The obtained relationship is different from the broadband scaling of Ide et al. (2007), which indicates the difference in physical process between small-scale LFEs and large-scale slow events. This result suggests that LFEs do not evolve to large slow earthquakes but they are slow phenomenon with small size triggered by the slow earthquakes with large size such as slow slip events driven by aseismic diffusion. In addition, despite the same moment-duration relationship, we observed that the moments of LFEs are three orders of magnitude smaller than those of ordinary earthquakes with similar durations. We interpret that the rupture velocity and/or stress drop of LFEs smaller than ordinary earthquakes cause the difference in source durations. The reasonable ranges of the rupture velocity and stress drop are 0.1–1 km/s and 2 kPa–1 MPa, respectively.